The present invention relates to a sensor for detecting an osmolarity of a specimen solution, and more particularly, to a sensor having a standard solution separated from the specimen solution by a membrane permeable to a vapor of a solvent and neither the solvent itself nor the solutes therein. The present invention is suitable for applications involving measurement of an osmolarity of a solution, including measuring an osmolarity of urine for the purpose of determining a level of kidney function.
An understanding of osmosis is necessary for the comprehension of the present invention and is therefore briefly reviewed. If two aqueous solutions of identical composition, but different concentrations, are separated from one another by a membrane permeable to water, but not to dissolved solutes of the solution, the water is transferred across the membrane to a side of denser concentration. At equilibrium, sufficient water is transferred so that the concentration of the solute on both sides of the membrane is identical. This passage of water in the direction of greater solute concentration is called osmosis.
The tendency of a solvent to pass through a semipermeable membrane is characterized by an osmotic pressure of the solution. For example, if a volume of fluid on the denser side of the solutions is in a contained space and not free to expand, because its capacity was limited by rigid walls, a pressure is developed in the contained space. An amount of pressure required to prevent an entry of water is called the osmotic pressure.
It is not strictly correct to refer to an osmotic pressure as being exerted by solutes, for example glucose, plasma proteins, or electrolytes, since solutes do no act by exerting a negative pressure across the membrane. Instead, it is a property of water which exerts a vapor pressure at any given barometric pressure and temperature. In that respect, water follows the gas laws. A boiling point of an aqueous solution is raised by solutes and a freezing point is depressed. This information serves as the basis for apparati used to measure the osmotic pressure of solutions.
If an aqueous solution is separated from pure water by a membrane, a potential pressure difference between the two phases is created and equilibrium is achievable by three mechanisms. In a first mechanism, an equal concentration of solute on both sides of the membrane may be established where the membrane is permeable to the diffusible solute. In a second mechanism, where the solute is not diffusible, water passes through the membrane thereby diluting the solution by osmosis and allowing a change in pressure and concentration on the respective sides to achieve an equilibrium of the system. Finally, a third mechanism utilizes an application of external pressure upon a solution, to oppose the osmotic pressure of the water, creating an opposing hydrostatic pressure preventing the movement of the water.